The present invention relates to silicon-on-insulator (SOI) substrates, and more particularly to a separation by implantation of oxygen (SIMOX) process in which internal thermal oxidation (ITOX) is sufficiently controlled so as to improve the circuit yield of the SOI substrate. The circuit yield can be increased in the present invention by improving the quality of the buried oxide (BOX) region and/or by decreasing the deep divot density.
During some SIMOX processes such as described, for example, in U.S. Pat. No. 5,930,643 to Sadana, et al., extensive internal thermal oxidation (ITOX) takes place in the oxygen implanted region of the Si-containing substrate during high-temperature (on the order of about 1300xc2x0 C. or higher) annealing in an oxidizing ambient.
FIG. 1 is a graph of ITOX thickness (xc3x85) vs. surface oxidation thickness (xc3x85) for typical SIMOX processes in which ITOX is occurring. Specifically, FIG. 1 shows how ITOX thickness depends on the surface oxide thickness formed during high-temperature annealing. As is shown, ITOX thickness increases with increasing surface oxide thickness.
Internal therm oxidation has both beneficial and detrimental effects on the quality of low-dose SIMOX processes. The beneficial effects that are obtained from ITOX include: (i) formation of a thermal oxide cap layer over the implanted oxide thereby increasing the overall thickness of the BOX region; (ii) improvement in the continuity of the BOX; and (iii) improvement in the electrical quality of the BOX region as measured by its breakdown field.
The detrimental effect of ITOX includes the formation of deep divots in the superficial Si layer (i.e., top Si layer that overlays the BOX region) of a SOI substrate which could be a device killer and impact circuit yield adversely.
The prior art SIMOX processes in which ITOX is a factor do not provide adequate control of this phenomena. Control of ITOX is important for various reasons. For example, the quality of the BOX region relates directly to the yield of microprocessors and memory devices such as dynamic random access memories (DRAMs) and static random access memories (SRAMs). Lowering the quality of the BOX region (i.e., lower the breakdown voltage) lowers the circuit yield. Similarly, the higher the deep divot density, the lower the circuit yield is. Both of these phenomena are directly related to ITOX which may occur during the high-temperature annealing step of conventional SIMOX processes.
In view of the above drawbacks with conventional SIMOX processes, there is a continued need for providing a new and improved SIMOX process in which ITOX is sufficiently controlled so as to improve the circuit yield of the SOI substrate.
One object of the present invention is to provide a SIMOX process in which ITOX is sufficiently controlled so as to be capable of adjusting the BOX thickness formed during the process.
Another object of the present invention is to provide a SIMOX process in which ITOX is sufficiently controlled so as to improve the quality of the BOX region.
A further object of the present invention is to provide a SIMOX process in which ITOX is sufficiently controlled so as to substantially decrease or eliminate the deep divot defects that are present in the top, i.e., superficial, Si-containing layer of a SOI substrate.
A yet further object of the present invention is to provide a SIMOX process in which no additional steps besides implanting and annealing are employed in forming a BOX region within a Si-containing substrate.
An even further object of the present invention is to provide a SIMOX process in which conventional ion implants are employed, and control of ITOX is achieved by changing the ambient employed during the annealing step.
These and other objects and advantages are achieved in the present invention by providing a SIMOX process in which ITOX is controlled by adding a chlorine-containing carrier gas to the annealing ambient during high-ternperature annealing.
Specifically, the present invention provides a method to control the quality of the BOX region formed during a SIMOX process, and to substantially reduce or eliminate deep divots in SOI substrates. The inventive method comprises the steps of:
implanting oxygen ions into a surface of a Si-containing substrate; and
annealing said Si-containing substrate containing said implanted oxygen ions at a temperature of about 1000xc2x0 C. or above and in a chlorine-containing ambient so as to form a buried oxide region that electrically isolates a superficial Si-containing layer from a bottom Si-containing layer.
The method of the present invention, which employs a chlorine-containing ambient during annealing, provides SOI substrates having a high quality (in terms of structural and electrical properties) BOX region and a superficial Si-containing layer that has a reduced number of divot defects present therein.
The term xe2x80x9chigh structural qualityxe2x80x9d is used herein to denote a SOI substrate which has little or no etch pitch density (less than about 1E5 cm2); little or no top or bottom superficial Si/buried oxide roughness (surface roughness of less than about 100 xc3x85 as measured by TEM (transmission electron microscopy); a low HF-defect density (less than about 1 cm2); and a low surface roughness (of about 6 xc3x85 root mean square).
The term xe2x80x9chigh electrical qualityxe2x80x9d is used herein to denote a structure wherein the buried oxide breakdown field is high (greater than about 5 megavolts per cm); the buried oxide minibreakdown voltage is high (greater than about 35 volts); the buried oxide leakage at a given voltage is low (less than about 1 nanoAmp); and the buried oxide short density is low (less than about 1 cmxe2x88x922).